Perhaps the confusion is coming from OP's activity having increased at the start of the most recent period, given that he already had made more than 42 posts at the time - he was probably expecting it to increase as he made 14 new posts through the period.

Also, yes, activity is updated every 30 minutes; that is, activity is limited not only by each 2 week time period, but also by your number of posts - so, if you have fewer posts than your maximum possible activity, every 30 minutes the forum will check to see if you have made new posts and update your activity accordingly. But that is no longer relevant here, as your post count exceeds your activity.